Among the RFID modules, Stronglink SL018 is the lowest cost RFID module that I can get which supports read and write. SL018 is a 13.56MHz RFID module, it supports Mifare 1K, Mifare 4K and Mifare Ultralight. The specification are as follow:

If you download the Raspbian image from Raspberry Pi official homepage and write it to the SD card, the SD card partition is fixed to 2GB, meaning that only 2GB is accessible no matter how big your SD card are. You must manually re-size the SD card partition in order to increase the available storage capacity.

App Inventor is an application use to develop Android application without programming. Its graphical interface is very similar to Scratch that allow users to drag and drop visual objects to develop application which runs on many mobile devices.

Google released App Inventor on December 15, 2010 and terminated as the Google product on August 2011. App Inventor is maintained by Massachusetts Institute of Technology and the code is become "Open Source".

Raspi-Config is a tool to help you to configure Raspberry Pi, it is a bit similar to adjust the computer BIOS settings. Raspi-config runs automatically when you boot your Raspberry Pi and you can access this tool anytime anywhere.